Rheumatoid arthritis (RA) is an incurable autoimmune disease that poses a significant health threat in the U.S. Traditional approaches for RA treatment, e.g. non-steroid anti-inflammatory drugs, often cause adverse reactions and have limited efficacy, requiring the development of new therapeutic strategies. Accumulating evidence supports the involvement of Toll-like receptors (TLRs) in RA, as evidenced by increased TLR expression and responsiveness in RA patients and in mouse models of the disease, and decreased RA severity in TLR2-/-, TLR4-/-, and myeloid differentiation primary response protein (MyD88)-/- mice. However, the molecular basis of exaggerated TLR responses in RA is unclear. Interleukin-1 receptor-associated kinase (IRAK) 4 is a central kinase used by all TLR utilizing the MyD88 pathway. Based on excessive TLR signaling in RA, the role of TLR-MyD88-IRAK4 signaling axis in these responses, and association of IRAK1 polymorphisms with RA in humans, it is possible that exuberant activation of IRAK4 underlies RA development, and regulation of IRAK4 activity could represent a novel therapeutic modality. We hypothesize that increased IRAK4 activation, due to distorted control by positive (Pellino-1) and negative (IRAK-M, Pellino-3b) regulators, is a critical driver of RA and that attenuation of IRAK4 activity will mitigate disease-promoting TLR pathways. This hypothesis will be tested in the following Specific Aims: 1. Define the role of IRAK4 in eliciting disease-associated inflammatory mediators in Mfs from RA patients and during progression of inflammatory arthritis in mouse models; 2. Engineer IRAK4 peptide antagonists and test their ability to inhibit inflammatory arthritis in mice. This exploratory/developmental R21 project will provide new proof-of-principle results to uncover the role of altered regulation of IRAK4 activation in the pathogenesis of RA and exploring the utility of novel cell- permeable IRAK4 peptide antagonists to attenuate arthritis in mice. It will pave the way for a future RO1 proposal aimed at in-depth comprehensive translational research in RA patients and mouse models of arthritis and provide the conceptual framework for rational design of inhibitors of IRAK4 activation as a new therapeutic modality for RA. These advances would be of key importance for basic immunology of RA and other autoimmune diseases and for improving public health of RA patients in the U.S.